# Homework Help: A small tiny electron problem

1. Sep 17, 2008

### Aerosion

1. The problem statement, all variables and given/known data

Okay, three electrons are arranged in a right triangle-like shape. The electron where the right angle goes is +2e, and the other electrons are +e. And at the half point of hte hypotenuse there is a point P and they want hte magnitude of the net electtric field.

They're saying that the two +es cancel out. How do they cancel out if they're both positive?

2. Relevant equations

3. The attempt at a solution

2. Sep 17, 2008

### Nick89

First of all, electrons are not +e charges and there is no such thing as a "+2e electron". Electrons are negatively charged particles with a charge of -e.

I assume instead of electrons there are simply 'charges' of +e and +2e, it does not matter what causes these charges.

Anyway, they cancel out because they are both positive. Suppose you have a testcharge of +1 C at point P. Also suppose one of the +e charges is called charge 1 and the other is called charge 2 (does not matter which is which).
The force the test charge feels due to charge 1 will push it away from charge 1, and hence, towards charge 2.
The force the test charge feels due to charge 2 will push it away from charge 2, and hence, towards charge 1.
The net force is therefore zero, both forces cancel each other out.

The same goes ofcourse for the electric field at point P but it is easier to explain in terms of force.

If one charge (let's say charge 1) however was negative (-e) then:
Charge 1 will attract the test charge (which is positive) towards charge 1, and away from charge 2.
Charge 2 will repel the test charge towards charge 1 aswell, so they don't cancel!